6. General characters of cephalochordata
• Marine, widely distributed in
shallow waters.
• Cephale=head, chord=cord
• No well defined head
• Mostly sedentary and buried with
only anterior body end, projecting
above bottom sand.
• Body small, 5-8cm long, slender,
fish-like, metameric and
transparent.
7. General characters of cephalochordata
• Body has trunk and tail
• Paired appendages lacking. Median fins present.
• Exoskeleton absent. Epidermis single-layered.
• Muscles dorso - lateral, segmented into myotemes.
• Notochord rod-like ,persistent extending from rostrum to tail and hence
the name cephalochordate.
• Coelom- enterocoelus.
• Digestive tract complete.
• Respiration through general body surface.
8. • Circulatory system well developed.
• Excretion by protonephrida with solenocyte.
9. • Nerve cord dorsal, tubular, without ganglia and brain. Dorsal and ventral nerve roots
separate.
• Sexes separate.
• Fertilization external in seawater.
10. FOSSIL HISTORY
• PIKAIA- Oldest known cephalochordates .
• Middle Cambrian period
• Anatomical features more or less same as Branchiostoma except a pair of
sensory tentacles that are found at the end of the body
11. • GEOGRAPHICAL DISTRIBUTION- Seas of temperate and tropical regions.
• SIZE- 50-70 mm.
• INDIAN CEPHALOCHORDATES- Branchiostoma lanceolatum, B.indicum, B.
bekheri (tamilnadu), B.lanceolatum have been recorded from the Indian
coast.
12. classification
• 35 species
• CLASS- leptocardii
• FAMILY- Branchiostomidae
• 2 genera- Branchiostoma and Asymmetron
• Asymmetron differs from branchiostoma in having unpaired gonads on the
right side of body and asymmetrical metapleural folds.
• Branchiostoma- 8 species, Asymmetron – 7 species.
13. PRIMITIVE CHARACTERS OF
CEPHALOCHORDATA
• No specialized head.
• Absence of paired limbs or fins.
• Epidermis is made up single row of cells and a true dermis is absent.
• Paired sense organs such as eyes, ears and nose are absent. The receptors
present are of a primitive type.
• Eggs are small with almost no yolk. Blastula is hollow, spherical and single-
layered.
15. AFFINITIES WITH NON- CHORDATES
• AFFINITIES WITH ANNELIDA
• COMMON FEATURES:
i. Body bilaterally symmetrical
ii. Protonephridia with solenocytes.
iii. Well developed coelom
iv. Closed and similarly disposed blood vascular system
• OBJECTION:
i. Coelom is enterocoelic and not schizocoelic as in annelids.
16. • 3 basic chordate characters of cephalochordate are not present in
Annelida.
17. Affinities with mollusca
• Pallas (1778) first described the amphioxus as LIMAX LANCEOLATUS
considering it to be a slug.
• Ciliary mode of feeding & respiratory mechanism through water current
which are common features of the 2 groups may be due to the similar
mode of life.
• OBJECTION- anatomy is completely different
• Molluscs are unsegmented and their locomotory podium is also unknown in
cephalochordate.
19. Affinities with Echinodermata
• Asymmetrical body, enterocoelic coelom and similarly formed mesoderm.
• Some fossil calyx of echinoderms look similar to gill-slits of amphioxus.
• As in branchiostoma, ophiuroids have similar phosphogens (creatine
phosphate) . But all these similar features may be because of a very remote
common ancestry of a 2 groups.
20. Chordate affinities
• AFFINITIES WITH HEMICHORDATA
i. PHARYNGEAL apparatus with numerous gill slits and gill bars.
ii. Filter feeding mechanism
iii. Respiratory mechanism
iv. Enterocoelic coelom
v. Numerous gonads without gonoducts.
21.
22. objections
• Muscles in hemichordate are unsegmented.
• Nervous system distinctly of non- chordata type, gill-slits dorsal in position
instead of lateral.
• Postanal tail is lacking
24. AFFINITIES WITH UROCHORDATA
• SIMILARITIES
i. BRANCHIOSTOMA AND HERDMANIA ARE REGARDED TO BE VERY
CLOSELY RELATED BECAUSE OF primitive ciliary feeding and respiratory
mechanisms.
ii. Large pharynx bearing numerous gills slits, epipharyngeal groove ,
endostyle & peripharyngeal bands
iii. Ascidian larva having a continuous notochord, above it is a dorsal hollow
nerve cord, and a post-anal tail with median caudal fin without fin rays.
iv. Identical early stages (holoblastic cleavage, gastrulation ) of
development.
25. OBJECTIONS
• Adult urochordates are extremely degenerate and sedentary animals
having several features unrepresented in cephalochordate such as:
i. Body unsegmented
ii. Covered by a test made of cellulose
iii. Without nerve cord and hollow nerve cord
iv. With a liver
v. A well developed muscular heart covered by peritoneum.
vi. Without nephridia.
vii. Sexes united with hermaphrodite gonads.
viii. Larva undergoing retrogressive metamorphosis to become the adult.
These differences show that inspite of close similarities reflecting upon a
probable common ancestry, the cephalochordates are better evolved than
the urochordates.
27. Affinities with cyclostoma
• Ammocoete larva of lamprey and branchiostoma show a similarity in many
characters such as:
i. Elongated, slender, fish-like body
ii. Continuous dorsal median fin
iii. Mouth surrounded by an oral hood and
iv. Pharynx region having endostyle and gill slits
Besides these fundamental chordate characters, their adults show
metameric myotomes, persistent gill slits, velum and a postanal tail.
28. Affinities with other vertebrates
Besides cyclostomes, branchiostoma also resemble other vertebrates in
several ways, such as:
i. Metamerically arranged myotomes
ii. True coelom lined by mesodermal epithelium
iii. Postanal tail
iv. Midgut diverticulum comparable with liver
v. Well-formed hepatic portal system
vi. Similar arrangement of main longitudinal vessels with forward flow of
blood in ventral and backward flow in dorsal blood vessel.
29. objections
• Lack of head, paired limbs, skull, vertebral column, muscular herat, red
blood corpuscles, brain, specialized sense organ, gonoducts, etc.,
• In possessing nephridia, atrium, numerous gonads, asymmetry etc..
30. Systematic position
• Cephalochordates possess all the important chordate characters so that
their inclusion in the phylum chordata is beyond doubt and conclusive in
the phylum remains controversial. They are definitely more evolved than
the hemichordates.
WILEY(1894) started that amphioxus is a prototype chordate which can be
placed along the main line of chordate evolution. According to GARSTANG
(1928) and BERILL(1958) cephalochordates and vertebrates both evolved
from a neotenic ascidian larva which failed to metamorphoses. But this view
is no longer upheld. The specialized characters of branchiostoma indicate
that it is not on the direct line of evolution of chordata.
COSTA(1834) & YAREEL (1836) and more recently GREGORY (1936) consider
branchiostoma to be a modified and degenerate form of some jawless
vertebrates .
31. • The absence of craniate or vertebrate characters (head, cranium, skull, vertebral column,
special sense organ ) and the presence of protonephridia unknown in chordates , show that
it has a very primitive character.
32. conclusion
• Cephalochordates possess a peculiar admixture of primitive, degenerate as
well as specialized or secondary characters. As such it can not be placed in
the direct line of evolution of chordate. It is regarded to be a generalized
and primitive type of chordata very close to the ancestral vertebrate.
But, because of its many differences, it is not included in the vertebrate .
Instead, it is placed in an independent subphylum cephalochordata.
